The present invention relates to novel polymers which are poly(4-hydroxystyrene/N-substituted maleimides) as well as a process for the production and use thereof. This polymer has been found to be useful in the preparation of various compositions such as adhesives and coatings, as well as binder resins for photoresists. These polymers have the empirical formula ##STR2## wherein X is independently H, C.sub.1 to C.sub.10 alkyl or alkoxy, amino or halogen, and R is aliphatic, cycloaliphatic, or aliphatic heterocyclic, and wherein the molecular weight ranges from about 1,000 to 50,000.
It is known in the art, according to U.S. Pat. No. 4,663,268, to produce substituted maleimide copolymers similar to those described above but wherein the R group is aryl. This patent teaches the usefulness of the aryl substituted materials as high temperature resistant binders for photoresists. Binder resins for photoresists must have many specialized properties. For example they must be water insoluble, yet soluble in aqueous alkaline solutions. They must be able to withstand the processing steps for integrated circuit production and must not interfere with the photosensitive characteristics of the oquinone diazide photo-sensitizers with which the binder is usually admixed. U.S. Pat. Nos. 4,298,720 and 4,100,140 describe certain bismaleimides. U.S. Pat. No. 4,663,268 and EP 0 187 517 describe certain maleimides having N-aryl substitution. EP 0 140 273 describes photoresists employing certain maleimide containing copolymers. Japanese patents 79016995; 61218607 and 54101200 describe copolymers of vinyl phenols and maleic acid anhydride. U.S. Pat. No. 4,525,536 describes copolymers of aromatic maleimides, maleimides and vinyl aromatic monomers.
It is desired in the art to provide binder resins for photoresists which have a high glass transition temperature (Tg) so that etch-resistance is enhanced. While polymers of high Tg are certainly known, most cannot be used for photoresists due to disadvantageous side properties.
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,475, 4,115,128 and 4,173,470. These include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired. Polyvinyl phenols are taught in U.S. Pat. Nos. 3,869,292 and 4,439,516.
The resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble that the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
In most instances, the exposed and developed substrate will be subjected to treatment by a substrate-etchant solution. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation. Thus, an etched pattern can be created on the substrate which corresponds to the pattern on the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of the photoresist on the substrate produced by the method described above is useful for various applications including as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, and resist adhesion.
Resist resolution refers to the capacity of a resist system to reproduce the smallest equally spaced line pairs and intervening spaces of a mask which is utilized during exposure with a high degree of image edge acuity in the developed exposed spaces.
In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small line and space widths (on the order of one micron or less).
The ability of a resist to reproduce very small dimension, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist.
Photoresists are generally categorized as being either positive working or negative working. In a negative working resist composition, the imagewise light struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The light struck parts are rendered soluble in aqueous alkali developers. While negative resists are the most widely used for industrial production of printed circuit boards, positive resists are capable of much finer resolution and smaller imaging geometries. Hence positive resists are the choice for the manufacture of densely packed integrated circuits.
Hence the invention provides novel polymers and photographic compositions employing these polymers. In one important aspect of the invention, it has been found that photoresists which employ the aforesaid polymer as a binder resin show comparable development and dissolution properties to phenol formaldehyde novolak resins but an advantageously lower dissolution rate than polyvinyl phenols which are also known for this purpose. Polyvinyl phenols have the disadvantage of faster dissolution, which makes image differentiation more difficult to attain. However, they are capable of withstanding much higher processing temperatures, on the order of about 200.degree. C., than the novolaks. The poly (4-hydroxy-styrene/N-substituted maleimides) of this invention demonstrate a dissolution rate comparable to the novolaks and can withstand processing temperatures comparable to the polyvinyl phenols.
In addition, these resins have an exceptionally low u.v absorptivity in the spectrum range conventionally used to expose photoresists. They are essentially non-absorbing at 300 nm and above, and have an extremely low absorptivity in the 254-270 nm range. Thus these resins are advantageous for resists exposed in the u.v. and especially in the deep u.v. part of the spectrum. This is in contrast to N-aryl substituted maleimides which disadvantageously absorb u.v. radiation in the 256-270 nm range.